Because a Writing score of 18 is only in the 58th percentile (meaning you did as well as or better than 58% of students who took the Writing test), it may negatively affect your chances at schools that require ACT Writing. Your composite score is either at (for Northwestern and UChicago) or above (for UC Berkeley, University of Illinois Urbana-Champaign, and UMich) the 75th percentile score for all of these schools, so you certainly shouldn't dismiss these schools as possibility offhand. However, of the schools that you listed, only the University of Illinois at Urbana-Champaign and University of Chicago don't require ACT Writing scores, which means that you might have a tougher battle when applying to UMich, UC Berkeley, or Northwestern.
It unfortunately isn't possible to send your ACT scores from the October ACT test date without also sending your ACT Writing score - if you take ACT Plus Writing, all of your scores are automatically included in your score report. The only way to get around that would be if you'd already taken the ACT without Writing and wanted to send in scores from that test date.
So should you take the ACT again? It depends on if you think you'll be able to put in the time to and effort increase your Writing score. If you have the time to practice the section and hone in on where you went wrong on the October ACT, then it is absolutely worth it. If you were shocked at your low Writing score and think there might've been an error, you should consider getting it hand-scored - it's $50 which you get refunded if there was an error, and the score is only adjusted in your favor (if the graders find you should've gotten a lower grade than you were initially given, your score won't be adjusted to a lower score). $50 is a hefty chunk of change, but it's still less expensive than having to take the ACT with Writing again.